1. Field of the Invention
The present invention relates to systems for sensing rotation that operate upon the interaction of counterpropagating beams of light within a cavity. More particularly, this invention pertains to a system for simultaneously sensing rotation about three orthogonal axes in which an axial magnetic field is imposed simultaneously upon the gain region within each of three lasing cavities.
2. Description of the Prior Art
U.S. patent application Ser. No. 115,018 of Graham J. Martin filed Oct. 28, 1987 entitled "Split Gain Multimode Ring Laser Gyroscope and Method" discloses a clear path, undithered multioscillator ring laser gyroscope. To a large extent the device and method of that invention provide an improvement over previous multioscillator designs by utilizing the concept of selected mode repression applying an axial magnetic field to the gain medium in a non-planar cavity, thus avoiding any intracavity elements.
Multioscillators belong to a class of ring laser gyroscopes in which stability problems are minimized by allowing four modes to lase within the device's cavity. This creates, in effect, two gyro beam pairs, one left circularly polarized and the other right circularly polarized. The lasing modes of a conventional multioscillator are configured so that two gyros, each comprising a pair of counterpropagating beams, simultaneously exists within the (single) cavity. The resulting sum of the beat outputs provides a signal that is doubly sensitive to input rotation and substantially insensitive to Faraday bias changes.
The type of multioscillator described in the referenced patent application comprises a clear path Sagnac ring rotation sensor that includes means for adjusting the gain medium to provide a frequency shift between selected gain curve centers. Such a frequency shift between the centers "suppresses" the lasing action of selected modes in the cavity to prevent frequency locking. The actual lasing frequencies of the cavity modes are not substantially changed by this frequency shifting.
The described multioscillator concept provides, in effect, a non-reciprocal bias in a four mode laser gyro by utilizing a large axial magnetic field without incurring the disadvantages of prior multioscillator designs such as the well-known ZEELAG (Zeeman Laser Gyro). Furthermore, such a device has such a large bias that back scatter effects become secondary.
Navigation systems must measure space-dependent variables, such as rotation, with respect to (or about) a set of three orthogonal axes. The realization of the many advantages of a multioscillator or any other rotation sensor, ring laser or otherwise, must address problems inherent in attempting to achieve a practical device that is simultaneously sensitive to rotating about three measuring or input axes. The design of a navigation system that is sufficiently compact and realizable in a manufacturing sense is beset by numerous difficulties. In the operation of a ring laser, the chosen fill gases must interact with applied electrical fields to produce lasing action. Thus the design of any ring laser gyroscope must provide for the positioning of anodes and cathodes in addition to properly locating mirror faces and internal bores.
Additional design problems are posed by a device whose operation relies upon the generation of current flows in a gaseous medium. Unavoidable gas flows within a laser cavity can prove quite deleterious to the operation of the device. So-called Langmuir flow effects can degrade laser performance considerably, producing inter alia unwanted thermal bias. Such effects have been compensated to varying extents in some single axis devices by the symmetrical placement of a plurality of electrodes about the body of the instrument. Generally, this implies the use of numerous electrodes. See, for example, the United States patents of Dorschner et al. (U.S. Pat. No. 4,229,106) and Smith et al. (U.S. Pat. No. 4,585,501).
The United States patents of Stiles et al., (U.S. Pat. No. 4,477,188) and Simms (U.S. Pat. No. 4,407,583) disclose the incorporation of three planar gyro cavities into a single block. The expansion of a ring laser concept to a unit for measuring rotation about three orthogonal axes necessarily complicates the problem of providing a suitable arrangement of electrodes. The Stiles et al. device utilizes six anodes and two cathodes while the Simms apparatus includes six anodes and a single cathode. The use of a considerable number of electrodes substantially complicates instrument design. Each electrode must be sealably secured to (or within) the gyro frame in such a manner that the device remains airtight. This may add significant difficulties in fabrication.
The physical size of the electrodes also complicate design. A large number of electrodes will consume a correspondingly-large percentage of the frame's surface mounting area. The size and shape of the block-frame may not be sufficiently reducible to prevent arcing or other unwanted electrical interactions. Thus, the design of a ring laser rotational rate sensor that is sensitive to rotation about three orthogonal axes is significantly complicated by unavoidable effects of gas flow.
In addition to the problems associated with placement of electrodes, the realization of a triaxial multioscillator in accordance with the teachings of the above-referenced patent application is further complicated by the requirement of an axial magnetic field for adjusting the separation between the centers of the gain media within each of the rotation-sensing cavities of the multioscillator. The single axis device disclosed in the referenced patent application alternately employs difficult-to-machine frame cutout regions and six-post magnet arrangements to encompass the gain region as required. Such designs are complex in the case of a single axis gyro. Their extension to three axes, even if possible, would result in a device of extreme complexity and cost. Undoubtedly, the extrapolation of such concepts to a triax design would introduce interactions between the axial fields for the three axes that could result in error-causing transverse components, an effect particularly noticeable in smaller path length designs.
The capabilities (i.e. sensitivity) and price of a triaxial rotation sensor are functions of the size of the block-frame. Any design that demands added surface area for separation of electrodes necessarily adds to the cost of the instrument. Such added cost partially defeats the compactness advantages of a three axes-in-one block device and can render the design inappropriate for single use applications, such as guided missiles, where the premium is on economy and accuracy is not critical.